This invention relates to processes for recording a signal on a disc and, more particularly, to processes for recording on a disc a wide-band signal which may be subsequently be read by optical reading techniques. This technique is known as the video-disc technique and is especially useful for the recording of television programmes.
The invention also relates to the apparatus used for carrying out these recording processes and to the discs obtained by these processes.
In general, television signals to be recorded do not correspond to a single base signal, but more often to several separate signals which have to be multiplexed. Thus, an ordinary television signal generally contains three separate signals, namely a luminance signal, a chrominance signal and a signal representing the sound.
These signals cannot be multiplexed simply by mixing, and thus various encoding processes have been proposed.
One of these processes is described in U.S. Pat. No. 4,068,259. In one embodiment of this process, three carriers S1, S2 and S3 are respectively frequency-modulated respectively by the three luminance, chrominance and sound signals. Since the band widths of these signals are very different, different amplitudes have had to be adopted for these carriers, in order to distribute the energy equally among them and to minimize intermodulation. The three carriers thus modulated are added and then amplitude-limited. The spectrum of the resulting composite signal comprises a large number of spectral lines, the amplitudes of the three main spectral lines corresponding to the three unmodulated carriers being related to the initial amplitudes of those three carriers.
The composite signal thus obtained is recorded on a disc in the form of marks of variable length and spacing distributed along a spiral track.
The disc thus obtained, or its copies, may then be read at will in a reader which includes an optical reading head which illuminates the recorded track with a light beam and which translates the recorded variations in spatial distribution introduced into the beam by the track into variations of an electrical reading signal.
Like any physical device, a reading head such a this has a limited and non-linear transfer function. FIG. 1 shows the output level A expressed as dB of the reading signal in a function of the spatial half-period l expressed in .mu.m, this spatial half-period l being defined as the length of a mark for a constant form factor and equal to 1/2, i.e., when the mark are separated by a distance equal to their length. This curve applies to a reading head provided with a lens having a relative aperture (the most important parameter) of 0.40.
Since the disc is rotated at a constant speed both during recording and during reading, the spatial frequencies corresponding to the three spectral lines increase regularly as the track moves nearer its centre, the reading radius becoming increasingly shorter. Co-relatively, the absolute and relative amplitudes of these lines in the reading signal change in accordance with this curve. The relative levels which had been defined during recording so as to limit the intermodulation, while at the same time maintaining and adequate signal-to-noise ratio, are no longer observed, resulting in degradation of the reconstituted image after processing of the reading signal.